The invention relates to the oxidation of starch.
The oxidation of starch is usually carried out in order to reduce the viscosity of the starch in solution or dispersion. During the oxidation reaction, starch molecules are broken down, yielding molecules having a decreased molecular weight.
Oxidized starches have found many applications in industry. Examples of common applications include the use of oxidized starch in the paper industry, for instance in coatings or surface sizing, the adhesive industry, the textile industry, and the food industry.
The preparation of oxidized starches is conventionally carried out by oxidation with an alkali metal hypochlorite, which is a relatively cheap oxidizing agent. The oxidation of starch using an alkali metal hypochlorite is extensively described in the literature (see among others xe2x80x9cModified Starches: Properties and Usesxe2x80x9d, O. B. Wurzburg, CRC Press Inc., 1987).
Although the oxidation of starch using an alkali metal hypochlorite is an efficient reaction, leading to high yields of desired product in a relatively short reaction time, it has some disadvantages. One disadvantage is that a large amount of salts is produced, in particular chloride salts, during the oxidation. Other disadvantages are the risk of chlorine formation during the reaction and of the occurrence of unacceptably high AOX levels.
In order to overcome the problems associated with these disadvantages, attempts have been made to find an alternative oxidizing agent. One alternative oxidizing agent that has received a significant amount of attention is hydrogen peroxide.
As early as in 1933, in the German Patent 738909, a process has been described wherein starch is oxidized using only hydrogen peroxide as oxidizing agent. The reaction is performed under semi-dry conditions at a temperature below 60xc2x0 C. The obtained oxidized starch product was intended to be used in puddings. It has meanwhile been found, however, that this process does not lead to a considerable extent of break down of the starch molecules in a reasonable reaction time. In other words, the disclosed process is very slow.
In order to improve the reaction rate of the oxidation of starch using hydrogen peroxide, it has been proposed to use metal catalysts in said oxidation reactions. In the international patent application WO-A-97/35888, it has been proposed to use metal based coordination complexes as catalysts in an alkaline slurry reaction of starch with hydrogen peroxide. The disclosed coordination complexes are based on a transition metal selected from groups VIIb, VIIb,, VIIIb or the lanthamide series of the Periodic Table. The complexes further comprise an organic ligand containing at least three nitrogen atoms which coordinate to the metal, and a bridging group.
From U.S. Pat. No. 3,655,644, the use of copper as a catalyst in the oxidation of starch using hydrogen peroxide is known. Although the patent mentions general amounts of copper of from 5 to 100 ppm, according to the examples at least 50 ppm needs to be used.
Disadvantages of the known processes for oxidizing starch using hydrogen peroxide and a metal based catalyst are that undesirably high amounts of catalyst (50 ppm) are required in order to be able to degrade the starch to a sufficient extent (intrinsic viscosity 0.1-1.5 dL/g) within a reasonable reaction time (less than 24 hours).
The present invention aims to provide a process for oxidizing starch using hydrogen peroxide, which process does not have the disadvantages of the prior art. More in particular, it is an object of the invention to provide a process for oxidizing starch using hydrogen peroxide, which process leads to a sufficiently degraded starch product in a relatively short reaction time.